Cross-site scripting (XSS) is a vulnerability that occurs when an application includes untrusted data in a web page without proper validation or encoding, allowing an attacker to execute malicious JavaScript in another user's browser. Once script execution is achieved, the attacker can steal session cookies, redirect users to phishing pages, modify page content, capture keystrokes, or perform actions on behalf of the victim. XSS consistently ranks among the most prevalent web application vulnerabilities.
How It Works
XSS manifests in three primary forms. Reflected XSS occurs when user input from a request is immediately echoed back in the response without sanitization. An attacker crafts a malicious URL containing script code and tricks the victim into clicking it. When the server reflects the input into the page, the script executes in the victim's browser. This variant requires social engineering to deliver the malicious link to the target.
Stored XSS is more dangerous because the malicious script is permanently saved on the target server, typically in a database, forum post, user profile, or comment field. Every user who views the affected page executes the attacker's script without any additional interaction. A single stored XSS vulnerability in a popular feature can compromise every user who encounters the injected content.
DOM-based XSS occurs entirely in the browser without the malicious payload ever reaching the server. Client-side JavaScript reads data from an attacker-controllable source (such as the URL fragment or document.referrer) and writes it into the page using dangerous sink functions like innerHTML or document.write. Because the payload stays client-side, server-side security controls and WAFs that only inspect HTTP traffic cannot detect it.
Why It Matters
XSS impacts are frequently underestimated. Beyond cookie theft, XSS can be leveraged for complete account takeover, data exfiltration, internal network reconnaissance through the victim's browser, and distribution of malware. The prevalence of JavaScript-heavy single-page applications has expanded the DOM-based XSS attack surface significantly. Thorough security testing examines every input reflection point, evaluates the effectiveness of output encoding, and tests Content Security Policy configurations that serve as a defense-in-depth measure against script injection.
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